Have you wondered how your watch actually figures out how much 1 second spans?
The answer lies in the concept of piezoelectricity.
Piezoelectricity is the phenomenon where a potential difference (a voltage) is generated across a piece of piezoelectric material when mechanical stress is applied to it. To put it in simple words, when you squeeze a piezoelectric crystal, electricity is generated which you can harness and use. The reverse effect is also true. When you pass electricity through a piezoelectric crystal (like quartz), it stretches, contracts and vibrates. This is known as the inverse piezoelectric effect.
What actually happens in the piezoelectric material can be understood by looking at its structure; how atoms are arranged.
In piezoelectric crystals/materials, atoms are arranged in such a manner that, when mechanical stress is applied, they bump into each other and sort of get squeezed and distorted. This causes the electrons in those atoms to separate thereby creating a net positive and net negative charge between two ends of the crystal.
In the same way, when electricity is passed through the crystal, the charges oscillate and this causes the crystal to deform and modify its physical shape.
To understand better, consider the atoms in SiO2 (quartz) which are arranged in a somewhat hexagonal structure (not exactly hexagonal). The bonds between Si and O atoms aren’t exactly neutral. The atoms are polarized where Si has a slightly positive charge and O has a slightly negative charge. But overall, the crystal is always electrically neutral. The structure is squeezed when mechanical stress is applied. This causes the Si atoms to come closer at the top and the O atoms to come closer together at the bottom, hence creating localized areas of positive and negative charge. An electric field develops across these areas. This is the reason for the net potential difference across two ends of the crystal and the subsequent flow of current when the circuit is closed.
Random Fact: Piezoelectric effect was first demonstrated by Pierre Curie and Paul-Jacques Curie in 1880 using quartz and Rochelle salt. It was first put into practical use by Paul Langevin who used it in sonar during WW1.
Quartz watches are a very common use of this fascinating phenomenon to measure seconds, minutes and hours. A quartz crystal is supplied with a fixed electric current which in turn makes the crystal vibrate at exactly 32768 times in one second (inverse piezoelectric effect). This is highly accurate and reliable. These vibrations in turn generate pulses of electricity (piezoelectric effect) which are counted by counters in a microcontroller. For every 32768 pulses (one more than 111111111111111 in binary) one second is registered. A signal is sent to a motor which is connected to gears which then rotate the second hand. For every 60 seconds, the minute hand moves and for every 60 minutes the hour hand moves. And voila! You have your time!
Piezoelectricity is used in many areas apart from timekeeping. Here are a few examples:
- Barbeque lighters make use of piezoelectricity to generate a spark. When the trigger is pressed, the crystal is hit so hard and fast that it develops a large potential which creates an electric arc (rips off electrons from air to form super-hot plasma) to discharge into the metal surrounding it which has a lower potential. This is the spark that ignites the gas.
- Certain microphones use piezoelectric materials to detect sound. Vibrations in the air (which is nothing but sound) apply stress on the piezoelectric material from which electric current is generated as a result.
- Piezoelectric transducers are used in different sensors where a mechanical input like pressure is to be converted into electrical signals.
In general, this electric current generated can also be stored and used when required, essentially making piezoelectricity a useful way to generate renewable energy. Imagine if your shoe had a piezoelectric material embedded in it, you could charge a small battery just by walking. Although it would take a lot of walking to accumulate a considerable amount of charge, as the generated current is miniscule, it could still be of use to charge small batteries for low power embedded devices.
To sum it up, you can squeeze electricity from certain crystals/materials which is known as piezoelectric effect. You can also supply electricity to piezoelectric material to change their length/make them vibrate which is termed “the inverse piezoelectric effect”. It has all sorts of uses, especially in sensing pressure differences/stress applied and can also be used to generate and store energy albeit on a small scale to power very small embedded systems.
This marvelous phenomenon is one of those times in engineering when you just stop and go wow!
